In the past, processes designed to produce a high purity product stream at high recovery from feed streams containing up to 90 mole % of the desired component required multiple stage membrane systems or multi-bed Pressure Swing Adsorption ("PSA") units. The use of stand-alone membrane units to produce a very high purity stream; i.e. greater than 99%, was found to be inefficient since large membrane areas and power demands were required in order to achieve this high purity at a high recovery. PSA units, on the other hand, provided to be very efficient in producing a high purity stream from feed streams containing the desired gas at concentrations greaters than 70 mole %, but become less efficient for treating relatively low purity; i.e. less than 70% streams to yield a high purity product at high recovery.
U.S. Pat. No. 4,229,188 teaches a process for recovering hydrogen from a gas mixture containing hydrogen and normally liquid hydrocarbons. The feed stream is passed to a selective adsorption unit to initially separate the feed, and the purge stream from the adsorption unit is subsequently treated in a membrane separator to recover an additional amount of the desired component. The process design disclosed in this patent, however, is only an efficient scheme for feed stream that are highly concentrated in the desired components; i.e. have a concentration of greater than 70 mole % of the desired component.
U.S. Pat. No. 4,238,204 discloses a process for recovering a light gas in both high purity and high yield from a gas mixture containing said light gas and other components. The gas mixture is initially directed to a selective adsorption unit which produces a high purity light gas and a purged gas containing at least a portion of the light gas. The purged gas from the adsorption unit is subsequently passed to a membrane permeator selectively permeable to the light gas in order to recover the permeated gas comprising light gas of improved purity from said permeator and recycling said permeated gas to the selective adsorption unit. As with U.S. Pat. No. 4,229,188, this process scheme is also only suitable for feed streams which are relatively highly concentrated in the desired feed component i.e. greater than 70 mole %.
U.S. Pat. No. 4,398,926 discloses a process for recovering hydrogen from a high pressure stream having a hydrogen content of up to about 90 mole %. The feed stream is passed to a separator containing a permeable membrane capable of selectively permeating hydrogen. The separator is used to achieve a bulk separation of the desired hydrogen component from impurities contained in the gas stream. The separated hydrogen is recovered at reduced pressure and passed to a pressure swing adsorption system adapted for operation at reduced pressure. Additionally, the off gas from the separator is recovered essentially at the higher pressure of the feed gas stream, and at least a portion of this stream is throttled to a lower pressure and passed to the pressure swing adsorption system as a co-feed gas in order to increase the recovery of the desired component.